In recent years, a fuel cell system has gradually come into widespread use as a power supply which is possibly capable of significantly contributing to solving energy issues and environmental issues. The fuel cell system generates power in an electrochemical reaction of hydrogen with oxygen in a power generation unit called a cell stack.
The power generation unit of the fuel cell system has the characteristics that it is not possible to rapidly change the amount of power to be generated. For that reason, with the fuel cell system, surplus power is supplied to an electric heater which heats water in a hot water storage tank, thereby implementing power supply that follows a rapid change in load power (hereinafter also referred to simply as “load”) (see Patent Literature (PTL) 1, for example).